Considering how much chartplotters cost these days, it seems pretty worthwhile getting it fixed or at least giving it a go.
You say it powers up...obviously it goes through some startup sequence that you recognise, be it a beep, or the key lights come on or something along those lines.
Generally speaking it's likely to be something small. If you suspect moisture, open it up and examine it. The circuit board should have some kind of protective material on it, generally a varnish...check this is intact and there are no rust spots. Clean any rust if present. Leave it open overnight in a warm place, and come back to it and re-try. Lift and re-seat any removable connections. If it were me, I'd take an eyeglass to the circuit board, especially around the connection between the display unit and the main board. Chances are you have either a damaged connection or a solder joint that has cracked over time. When you consider the pounding these things take, it's not surprising. Car stereos suffer from similar problems.
Solder problems will show as either hairline cracks between a pin and the glob of solder, or the track having broken or lifted. Check everything.
Beyond this, all I can suggest is to pass it to Simrad themselves.